Chemical analyses of fluids, such as natural bodies of water, often involve sample collection and transportation of the sample to a remote test site. Collecting and transporting the sample may alter the sample's composition. For instance, the collection process may contaminate the sample, or a chemical reaction may occur that changes the composition. Collecting and transporting the sample is therefore inefficient, costly, and time consuming.
Moreover, discrete sampling characterizes the state of the fluid only at the time the sample is collected. Natural systems are not static; therefore, the dynamic interactions that occur in natural systems can only be surmised from samples taken at various time intervals.
Remote probes addressed a few of the problems associated with discrete sampling and transportation of the sample to a remote test site. Unfortunately, remote probes are typically limited to measuring gross properties of fluid systems such as temperature, conductivity, and suspended particulate loading.
The advent of fiber optics also addressed limitations apparent in prior analytical devices. For instance, optic fibers convey light to the sample itself, rather than the sample being conveyed to the light source. Furthermore, flexible optic fibers are not limited to specific configurations required in typical laboratory devices. Thus, optic fibers can be configured in transportable devices in manners previously inaccessible.
Bifurcated spectrophotometers are known and have been used to measure various properties of aqueous systems. Many of these devices measure properties of a fluid only through light-scattering or light-absorption techniques. Detecting analytes with light absorption techniques has several disadvantages as compared to detecting fluorescence For instance, the optimum color change associated with an analyte may require lengthy time periods to develop at room temperature, making "real time" measurements impossible. Also, the detection limit of absorption measurements is limited by the extinction coefficient of the colored complex, whereas a fluorescent emission signal can be increased by increasing the excitation energy.
U.S. Pat. No. 4,548,907 describes a bifurcated fiber optic device that includes a multi-wavelength light source and a photomultiplier connected to the light detecting arm. The common end of the optic fibers can be immersed in a fluid sample to measure the fluorescent emissions of a pH sensitive fluorophore. Carbon dioxide levels can also be detected with the device by measuring the pH change associated with the carbon dioxide-bicarbonate ion equilibrium.
Briggs' U.S. Pat. Nos. 4,564,598, 4,676,640, and 4,739,171, describe a bifurcated fiber optic system that measures particulate levels in fluid systems by monitoring fluorescent emissions. These patents also describe a flow tube that houses beads coated with a reagent such as a photosensitive dye.
Knight's U.S. Pat. No. 4,753,530 describes a device intended to be a transportable detecting device for analyzing aqueous media. The bifurcated or trifurcated optic system can measure properties of dilute fluid systems, including turbidity, absorption, reflection, fluorescence and phosphorescence.
The sensitivity and complex nature of typical analytical instruments limit their use to laboratory settings. For instance, spectrophotometers include sensitive electronic equipment and are therefore not suitably immersed in fluids during testing operations Even analytic devices that are compact and transportable require a skilled operator to attain accurate data. Thus, currently available devices are not easily translocated and are susceptible to damage from hostile environments.
Finally, prior art devices typically are designed to measure natural fluorescence rather than induced fluorescence. These devices are therefore limited to detecting species that are themselves capable of fluorescent emissions.
The aforementioned problems make the real time, in situ analyses of fluid systems, such as natural bodies of water, heretofore infeasible.